Liquid driven downhole drilling machines of this kind are often used with drill tubes that are added to one another and the thus formed drill string is rotated so that the drilling machine and thereby the drill bit is indexed between each impact of the piston hammer. The drill bit is angularly fixed in the housing. When deep holes are drilled, although the rotation of the upper end of the drill tube is continuous, the friction between the drill tube and the borehole wall will sometimes make the rotation of the lower end of the drill tube uneven. The drill tube will act as a torsion spring and instead of being indexed evenly between the impacts of the piston hammer, the drilling machine will not be turned while there are several impacts and then it will be rapidly turned. This slip-stick effect reduces the drilling rate and increases the drill bit wear.
In liquid driven downhole drilling machines, the power liquid is supplied through the drill tube and the return stroke of the piston hammer is retarded hydraulically which induces pressure spikes since the piston hammer will then force liquid out into the drill tube. This will result in high stresses and also in a reduction of power efficiency. Attempts have been made to have an accumulator in direct connection to the drilling machine but hitherto there is no good solution to this problem.